Positron Emission Tomography (PET) is one of the three major modern medical diagnostic approaches used as a non-evasive tool to detect tumors or other abnormal conditions of the body. A modern PET scanner may typically use a very large number of scintillating crystals as gamma-ray detectors with a relatively small x-y dimension, and a long z dimension. Typically on the order of 20,000 to 30,000 such scintillation crystals are used in a PET scanner detector ring to capture the two emitted gamma rays from each positron-electron annihilation event to thereby locate the location with high precision. It is possible to trace back the locations of emission and thus reconstruct the tumor image accordingly.
Scintillation is a process to capture the gamma-ray and convert it into visible light that, in turn, can be detected with a photodetector, such as photo-multiplier tube (PMT), photo diode, or more advanced silicon photo-multiplier (SiPM). Of course, to have effective detection, it is desired to have more visible photons generated by each gamma-ray capture event.
Because of the desire in high energy physics to detect various high energy particles, there have been extensive searches in the past century for more efficient scintillator crystals that generate more photons per gamma-ray capture. For PET scanners, it is desired not only to capture the gamma-ray, but also to know the position of capture to make the accurate image reconstruction. To do so, the scintillator crystals are normally cut into thin long rectangular rods packed into two dimensional array blocks. These array blocks are installed in the PET scanner to form a detector ring of different sizes. Depending on the specific application, patient or animals will be scanned through to detect the tumors inside their body.
The construction of the detector array block is important to the performance of the PET scanner. It is desired to select have scintillating crystals with high light yield and good stopping power to have efficient capture of the gamma-ray. Since there are a large number of crystals packed together, it is desired that the crystals are mechanically strong so that they can be cut and polished into such small thin rods. The typical physical size of these crystals varies from 6 mm down to 0.5 mm in cross-dimension and 30 mm down to 5 mm in length depending on the specific application. The size of the array block will also vary depending on the kind of photodetector used, the specific dimension and geometry of the photodetector and finally the specific scheme of detection.
In the traditional way of gamma-ray detection using PMT detectors, the scintillating crystal blocks are each typically built in a 12×12, 13×13, 14×14 or even larger array depending on the PMT arrangement. Each of the individual crystals are optically isolated to each other with high reflection films that covers five sides of the crystal surfaces except one end where the scintillating light will be emitted to reach to the PMTs. For example, a 14×14 array block will contain 196 crystal pixels which share only four PMTs. To have the emitted scintillating light reach to all four PMTs, a special designed light guide may be used between the array block and the PMTs. The principle used to locate the exact position of the emitting crystal is based on the calculation of the distributed light sharing ratio from these four PMTs. Since the PMT is a relatively expensive detector, to control the total cost of a typical PET scanner, it is desirable to use as few PMTs as possible, but at the same time it is also desirable to be able to accurately locate the position of the scintillating light source. At the present time, the 14×14 array has reached just about the detecting limit for the PMTs. Each PMT in this case will share forty-nine crystals. So for a full size scanner ring, a manufacturer may limit the total number of PMTs used to below 600 units to control the cost, but still have high enough image resolution.
To be able to have an accurate calculation of the emitting pixel position, all the four PMTs should be able to detect adequate amount emitting photons at the same time frame. This means that the scintillating crystal should desirably emit as many visible photons as possible with each capture of the incoming gamma ray. This is the very reason that there are extensive research efforts to find the best scintillating crystals. However, there is a limit on how one can find such a crystal with so much light emission. Even with good light emission, it may also be equally important to be able to channel the scintillating light to the end of the crystal so that it can reach the PMTs. Accordingly, there are also efforts to select the best reflecting film so that sufficient light can be reflected out at one end of a crystal.
To be able to capture as much of the emitting gamma rays from a patient's body, it is desired to have the detector ring packed with a maximum volume of the scintillating crystals. This means that one would to reduce the amount of volume for the reflecting material. So the reflecting film should be thin and effective. Over the years there are a number of materials that have been selected as reflectors. Here are listed of some of the well-known reflectors that have been used: liquid white paints made of MgO, TiO2 or BaSO4; solid powders of MgO, or TiO2; reflecting films, such as Teflon tape, Lumirror film or 3M Vikuiti Enhanced Specular Reflector (ESR) film. Such materials should also be compatible for ease of manufacture, especially for mass production. In the traditional PMT detector based array blocks, the choice of reflecting film is relatively forgiving, since the scintillating crystal size is usually not so small.
A number of patents disclose approaches to preparing the scintillator crystals and the various reflective materials used to make an array of such crystals. For example, U.S. Pat. Nos. 5,610,401; 8,481,952; 9,012,854, and 8,426,823 each discloses various crystal and packaging configurations.
There still exists a desire for better scintillation crystal performance, especially for radiation detectors, such as PET scanners.